The present invention generally concerns ring lasers and, more particularly, apparatus and methodology for reducing rotational bias errors in ring lasers.
Ring lasers have become widely used in applications related to the measurement of rotational motion and acceleration. A ring laser typically forms a closed polygonal optical path within which two or more laser beams propagate in opposite directions. This closed optical path may either be flat or, alternatively, non-planar. The ring laser has a sensing axis that passes through the closed optical path of the counter-propagating beams. For a flat, or planar, ring laser this axis is normal to the closed optical path. In non-planar ring laser configurations, the sensing axis may be a line normal to the projection of the optical path upon a plane.
When the ring laser is not rotating about the sensing axis, the optical paths of the counter-propagating beams have identical lengths. Rotation of the ring laser about the sensing axis, however, causes the effective path length to increase for the laser beams travelling in the direction of the rotation while the path length for the laser beams travelling in the opposite direction, counter to the sense of rotation, effectively decreases. This effective shift in the optical path lengths of the counter-propagating laser beams in the ring laser induces a measurable shift in the frequency of each of the beams. Thus rotational motion and acceleration of the ring laser about the sensing axis can be determined by measurement of these shifts in frequencies. To effectuate this measurement, a portion of each counter-propagating laser beam is typically extracted from the closed optical path of the ring laser. Beam combining optics are then employed to direct both beams co-linearly along a common path to a detector that senses a beat frequency resulting from the frequency shifts.
One major difficulty previously experienced in ring lasers is the disappearance of the frequency difference between the counter-propagating beams within the ring laser when the rotation rate of the ring laser is within certain limits. At low rotation rates this loss of frequency difference can produce a false indication that the ring laser is not rotating. Called lock-in, this effect arises from an optical coupling of photons between the counter-propagating beams within the ring laser. This coupling can result from backscatter of a portion of each beam off the mirrors defining the closed optical path of the ring laser. Each laser beam within the ring laser cavity thus includes a small component having the frequency of the beam propagating in the opposite direction, causing the resultant frequency of each of the laser beams to shift towards, or lock-in, the resultant frequency of the other counter propagating beam. At sufficiently low rates of ring laser rotation, the two counter-propagating laser beams are at the same frequency and lock-in occurs.
Lock-in has been substantially reduced with the development of ring lasers generally called multi-oscillator type ring lasers. In multi-oscillator ring lasers, a non-planar laser cavity is employed to obtain four laser modes from two pairs of counter-propagating beams. One of the pairs of counter-propagating beams is left circularly polarized, while the other pair of counter-propagating laser beams is right circularly polarized. Thus one of the left circularly polarized laser beam travels about the ring laser cavity in a clockwise direction with respect to the sensing axis of the ring laser while the other left circularly polarized laser beam travels about the ring laser cavity in an opposite, or anti-clockwise direction. Similarly, one of the right circularly polarized beams also travels about the ring laser cavity in a clockwise direction while the other at the right circularly polarized laser beams travels in an anti-clockwise direction. The frequency of the left circularly polarized pair of laser beams also differs from the frequency of the right circularly polarized pair of beams. In one type of multi-oscillator type ring laser, a Faraday element is also included in the optical path within the ring laser cavity to reduce sensitivity to lock-in. Application of a magnetic field to the Faraday element induces a slight frequency separation between the two counter-propagating left circularly polarized laser beams and between the two counter-propagating right circularly polarized beams. This frequency separation shifts the lock-in region to rates of rotation greater than those rates that are usually of interest.
Split gain ring lasers similarly employ a non-planar cavity configuration to obtain two pairs of left circularly polarized and right circularly polarized counter-propagating laser beams. The split gain ring laser, however, does not require a Faraday element to achieve a frequency separation between the two counter-propagating beams. Instead the gain curve of the split gain laser is divided into two separate maxima by application of a magnetic field to a plasma discharge region of the ring laser cavity. Each maxima of the gain curve will then amplify, or support lasing, in modes of only one helical orientation. For example, a left circularly polarized laser beam propagating in the anti-clockwise direction about the ring laser cavity has the same helical orientation as a right circularly polarized laser beam propagating in the clockwise direction. Thus one of the maxima of the gain curve for a split gain ring laser will amplify only the anti-clockwise propagating left circularly polarized laser beam and clockwise propagating right circularly polarized laser beam while the other maxima will only amplify the left circularly polarized laser beam propagating clockwise and the right circularly polarized beam propagating anti-clockwise. The strength of the magnetic field and, therefore, the range of frequencies within each maxima of the gain curve, can then be selected to support lasing at two separate longitudinal modes within the free spectral range of the ring laser cavity. This magnetic field strength is usually chosen to provide a frequency splitting of the two portions of the gain curve equal to the free spectral range of the ring laser cavity. Thus the anti-clockwise propagating left circularly polarized and clockwise propagating right circularly polarized laser beams will be of a different longitudinal mode from the clockwise propagating left circularly polarized and anti-clockwise propagating right circularly polarized laser beams. The resulting laser beam modes within the split gain ring laser are thus separated in frequency either by the longitudinal mode spacing of the ring laser cavity or by the geometrically induced reciprocal splitting of the laser cavity configuration. This frequency separation greatly reduces the susceptibility of the split gain ring laser to lock-in, even in comparison with multi-oscilator type ring lasers incorporating a Faraday element, and avoids the complications associated with placement of a Faraday element in the ring laser cavity.
The split gain ring laser is the subject of a co-pending patent application entitled "Split Gain Multi-Mode Ring Laser Gyroscope And Method" (inventor Grahm Martin), Ser. No. 07/115,018 filed Oct. 28, 1987. This co-pending application has been assigned to Litton Corporation, the assignee of the current application, and is under a secrecy order (Type I).
While the split gain ring laser enjoys less susceptibility to lock-in, other forms of rotational bias error have begun to predominate as limitations in the accuracy of the laser. One source of rotational bias error is caused by the re-injection back into the ring laser cavity of a small portion of each of the laser beams that are coupled out of the laser for beat frequency measurement. Re-injection of a portion of a laser beam mode back into the ring laser cavity in the same clockwise or anti-clockwise direction as the initial laser beam mode can induce a frequency shift in that mode within the ring laser cavity and, thus, can inject an error in the beat frequency measured by the detector. Accordingly, there still exists a need to reduce or eliminate such rotational bias errors. The present invention fulfills this need.